1. Field of the Invention
This invention relates to a method for pulling semiconductor crystals such as silicon crystals and, more particularly, to an apparatus having a double structure crucible which is divided into inner and outer chambers, and a method for controlling the resistivity and other crystal characteristics by means of the crystal pulling apparatus.
2. Description of the Related Art
Conventionally, the Czochralski technique (CZ technique) is a known method used to grow a rod-like semiconductor single crystal from melt in a crucible, and as is well known in the art, the impurity concentration distribution C in the longitudinal direction of the single crystal grown by use of this method can be expressed as follows: EQU C=kCo(1-G).sup.k-1
where k is the segregation coefficient of the dopant, Co is the initial impurity concentration of the melt, and G is the solidification ratio. When the value of k is low, the impurity concentration distribution in the longitudinal direction of the grown single crystal varies significantly, as a result of which, the yield of single crystal having the desired impurity concentration range (or resistivity range when the impurity is an electrically active material; i.e. the conductive impurity) may be significantly reduced.
With the aim of solving this problem, a floating type double crucible method, in which the surface level of the melt in an inner crucible is kept constant, has been proposed for use in growing single crystal of germanium or silicon, and is disclosed in the following document:
Solid-State Electronics Program Press 1963. Vol 6, pp. 163-167. Printed in Great Britain
GENERAL CONSIDERATIONS CONCERNING THE DOUBLE-CRUCIBLE METHOD TO GROW UNIFORMLY DOPED GERMANIUM CRYSTALS OF HIGH PRECISION
H.F.MATARE
Bendix Research Laboratories Division, Southfield, Mich.
(Received 10 September 1962; in revised form 8 November 1962)
The double-crucible method will now be explained in detail, with reference to FIG. 14. As is shown in FIG. 14, inner crucible 2 is arranged as a floating crucible inside outer crucible 1, with small hole 3 being formed in the bottom of inner crucible 2. When, for example, crystal 6 is pulled from melt 4 in inner crucible 2, the balance between the buoyancy of inner crucible 2 and the force of gravity is utilized to maintain the surface level h of the melt in the inner crucible. Further, the outer crucible is raised relative to the fixed inner crucible, so as to supply melt 5 from the outer crucible to the inner crucible in such a way that the surface level h of the melt in the inner crucible is kept constant. Assuming that the impurity concentration of melt 5 in the outer crucible is Co, and that the impurity concentration of melt 4 in the inner crucible is Co/k (k being the segregation coefficient), then in the pulling process, in which surface level h of the melt is kept constant, the concentration of impurity taken into pulling crystal 6 becomes Co, thus ensuring that the melt (pure silicon or germanium) and the impurity used for growing the crystal are always supplied in equal amounts from melt 5 in the outer crucible to melt 4 in the inner crucible. As a result, the impurity concentration of melt 4 in the inner crucible is kept at Co/k, and thus the impurity concentration of pulling crystal 6 is kept at constant value Co.
However, in the pulling process, some of the melt is consumed and the surface level thereof is lowered. After the bottom outer portion of inner crucible 2 comes into contact with the bottom inner portion of outer crucible 1, the impurity concentration cannot be kept constant, and thus the impurity concentration of crystal 6 will vary (i.e. increase) as the solidification ratio increases. More precisely, the impurity concentration can be kept constant only within the following range of the solidification ratio G: EQU 0.ltoreq.G.ltoreq.1-(h/H) (1)
where H is the initial surface level of the melt in the outer crucible, and h is the surface level of the melt in the inner crucible, to be kept constant during the pulling process. Therefore, in the case where the floating type double-crucible method is effected by using an impurity acting as a donor or acceptor to grow a crystal having a constant resistivity in the longitudinal direction, the resistivity can be kept constant only when the solidification ratio is less than 0.6 to 0.7, the resistivity varying significantly after the solidification ratio becomes higher than this value.
Another single crystal growing method is the floating zone technique (FZ technique), and according to this technique, it is possible to grow a rod-like single crystal having a constant impurity concentration in the longitudinal direction. However, it is generally the case that the distribution of the dopant impurity in the radial cross section of a single crystal obtained by way of the FZ technique is non-uniform in comparison with a single crystal obtained by way of the CZ technique. For example, in the case of a Si single crystal wafer of 5".phi., the growing direction (111) of which is represented by Miller indices, the in-plane distribution .DELTA..rho. of resistivity .rho. attained by resistivity measurement using four probes is 6 to 15% when the CZ technique is used, but reaches as high as 20 to 50% when the FZ technique is used. In this case, .DELTA..rho.=(.rho.max.times..rho.min)/.rho.min, the in-plane resistance variation .DELTA..rho.SR caused by the spreading resistance measurement being 10 to 20% in the CZ crystal, but reaching as high as 30 to 50% in the FZ crystal.
When the CZ technique is used to grow a silicon single crystal, oxygen having 1.times.10.sup.18 atoms/cc is introduced from the quartz crucible containing the melt into the crystal, while when the FZ technique is used, introduction of oxygen is suppressed to a minimum, since the melt is not in direct contact with the crucible. Since the introduction of oxygen into the silicon crystal tends to harden the wafer, therefore a wafer obtained by use of the FZ technique,--i.e. one containing less oxygen, and therefore more soft--tends to become warped during the heat treatment, and slippage occurs more easily than in the case where a wafer is obtained by way of the CZ technique.
In order to solve the problems associated with the CZ technique, the floating type double-crucible method, and the FZ technique; the inventors of the present invention have proposed a crystal pulling apparatus having a novel integral type double crucible structure (cf. Japanese Patent Application No. 61-221896 which corresponds to U.S. Patent Application, Ser. No. 091,947 filed on Sept. 1, 1987. The copending related Japanese Patent Applications are Japanese Patent Application Nos. 61-238034, 62-200839, and 62-229632). The present invention improves the invention of the above-quoted patent applications.